A hot water boiler of the kind set forth above, is described in the specification of Austrian patent application A8167/73 the name of Tirolia-Werke. Such boilers comprise watertight, vertical, cylindrical vessels connected by way of upper and lower piping connections, the vessels being made of metal and having connector stubs of said connections, projecting laterally from tubular portions of the vessels, at positions back from their ends.
The connecting stubs of the vessels are welded together to provide what is known as, a "flat" or a "shallow", hot water storage tank. Such metal vessels are difficult to produce and to weld together. Also, because of the location of said stubs, water does not flow through parts of the upper and lower ends of the vessels, unused space being therefore present therein. When the tank is heated, the water circulation is imperfect, so that considerable thermal stresses materialise, which may cause leakage at the supply and outflow connectors.
Also, the vessels need to be purpose built for particular tank capacities so that a substantial number of different size vessels must be held in stock.